Core-shell ABS copolymers and method for their preparation

ABSTRACT

Core-shell ABS copolymers having an SAN shell completely surrounding the diene core are provided by melt blending an aliphatic diene polymer with an SAN copolymer, said copolymer containing functional groups capable of reacting with the diene polymer. For example, the SAN copolymer may contain azide groups capable of reacting with the olefinic groups in the diene polymer, or both the SAN copolymer and the diene polymer may contain other functional groups capable of interreaction, such as acidic (e.g., maleic anhydride) groups in the diene and epoxy (e.g., glycidyl methacrylate) groups in the SAN copolymer.

This application is a division, of application Ser. No. 08/015,958,filed Feb. 8, 1993, now abandoned.

This invention relates to the preparation of core-shell copolymers, andmore particularly to the preparation of copolymers of aliphatic dienes,alkenylaromatic compounds and ethylenically unsaturated nitriles.

Various copolymers of aliphatic dienes, alkenylaromatic compounds andethylenically unsaturated nitriles are known. The monomers involved aremost often butadiene, styrene and acrylonitrile, and the polymersthereof are generically designated "ABS resins". They are most oftenproduced by initially preparing a polybutadiene seed latex andcopolymerizing styrene and acrylonitrile in emulsion in the presencethereof.

An intermediate copolymer which may be formed in this process,hereinafter designated "high rubber graft", is a copolymer at leastpartially in core-shell form in which the principal constituent of theelastomeric core is polybutadiene, prepared in emulsion as a latex. Astyreneacrylonitrile copolymer (hereinafter sometimes "SAN") is presentpartially as occluded domains in the polybutadiene and partially as ashell around the polybutadiene particles.

A typical high rubber graft comprises about 40-60%, typically about 50%,polybutadiene (all percentages herein being by weight) and about 40-60%SAN. About 35-45% by weight of the SAN, representing about 18-23% oftotal polymer, is chemically bound to the polybutadiene latex particles,either as a shell on the surfaces thereof or as an internal occludedmaterial; the remainder of the SAN is unbound.

It is also conventional to melt blend the high rubber graft withadditional SAN copolymer to produce a blend containing about 15%polybutadiene. Such a blend is the principal constituent of manyvarieties of commercial ABS resins, which may also contain othermaterials such as antioxidants.

The morphology of ABS resins may be studied by transmission electronmicroscopy. In a typical scanning operation, the ungrafted SANcontinuous phase is removed by dissolution in acetone and the remainingcore-shell particles are dispersed in a solution of polybutadiene intoluene. The toluene is removed by evaporation and the residue is moldedin a block of epoxy resin. Said block is stained with osmium tetroxideand thin sections are microtomed and imaged on a transmission electronmicroscope. The polybutadiene particles receive the stain and aredarkened, while the SAN portion remains light colored.

It is uniformly observed that a portion of the SAN forms occlusionswithin the polybutadiene particles, while the remainder forms anincomplete shell around said particles. Said shell typically has a"raspberry" configuration with droplet-shaped shell regions separated byareas of exposed polybutadiene.

Such shell-free regions are points of stress field discontinuity.Stresses in molded ABS articles are, in the ideal situation, transmitteduniformly through the shell to the elastomeric core particles, wherethey are dissipated. Stress field discontinuities, however, may causenon-uniform stress transfer and adversely affect the performance of themolded article. In addition, it is possible for diene polymer with anincomplete shell to agglomerate and be poorly dispersed in the externalSAN matrix.

It is desirable, therefore, to provide a method for forming core-shellABS and similar copolymers in which the shell completely, orsubstantially completely, surrounds the core particles. A method forproducing such resins is provided by the present invention.

In one of its aspects, the invention is a method for preparing acore-shell copolymer in which the shell is at least partially chemicallylinked to the core, which comprises melt blending (A) a compositioncomprising an aliphatic diene polymer with (B) a copolymer of analkenylaromatic compound and an ethylenically unsaturated nitrile, saidcopolymer B containing functional groups capable of reacting with saiddiene polymer.

The aliphatic dienes, alkenylaromatic compounds and ethylenicallyunsaturated nitriles employed in the present invention are well knowncompounds. Examples of dienes are butadiene, isoprene and chloroprene.Alkenylaromatic compounds include styrene, α-methylstyrene andvinyltoluenes. Ethylenically unsaturated nitriles include acrylonitrileand methacrylonitrile.

By reason of the commercial availability of butadiene, styrene andacrylonitrile as well as their particular suitability, they are themonomers usually employed in the present invention and reference willfrequently be made specifically to them hereinafter. It should beunderstood, however, that other suitable monomers may be substitutedtherefor when appropriate.

Component A according to the present invention is a compositioncomprising an aliphatic diene polymer. It is generally prepared inaqueous emulsion, in the form of a latex. Most often, butadiene is firstpolymerized to form a seed latex to which styrene and acrylonitrile aresubsequently added.

Thus, it is apparent that component A may also contain SAN copolymer,which may be at least partially chemically bound to the surface of thepolybutadiene particles. The aforementioned high rubber graftcompositions are generally preferred as component A.

Component B is a copolymer of said ethylenically unsaturated nitrile andalkenylaromatic compound, and is typically an SAN copolymer. It mostoften contains about 50-85% styrene units, based on total styrene andacrylonitrile units.

Copolymers suitable for use as component B may be prepared byart-recognized methods, including bulk and solution methods. Solutioncopolymerization is frequently employed.

For the purposes of the present invention, it is essential that the SANcopolymer contain functional groups capable of reacting with the dienepolymer. Various embodiments of the invention may be employed for thispurpose.

In one embodiment, the SAN copolymer additionally contains units capableof reacting with the olefinic groups in the diene polymer. Said unitsmay contain, for example, azide groups provided by incorporating in theSAN copolymer units derived from an ethylenically unsaturated azide suchas vinylbenzyl azide.

In another embodiment, the diene polymer and SAN copolymer contain otherunits which have functional groups capable of interreaction. Forexample, the diene polymer may contain acidic units derived from amonomer such as maleic anhydride, and the SAN copolymer may containepoxy units derived from a monomer such as glycidyl methacrylate.

The interreactive groups may be introduced by simultaneouspolymerization or by grafting. It is generally convenient tofunctionalize component B by simultaneous polymerization of styrene,acrylonitrile and the reactive monomer, and to functionalize component Aby grafting a suitable monomer onto the high rubber graft, principallyonto the diene polymer therein, most often in the presence of a freeradical initiator such as dicumyl peroxide. The proportion of monomerscontaining the desired functional groups is generally about 0.5-50%based on total component A or SAN copolymer, respectively.

According to the invention, components A and B as defined above are meltblended. Conventional batch or continuous melt blending operations maybe employed. Extrusion is frequently preferred by reason of itsparticular effectiveness and capability of continuous operation. It maybe performed in a single- or twin-screw extruder, typically attemperatures in the range of about 200°-300° C.

Transmission electron microscopy of the products prepared by the methodof this invention, after treatment as described hereinabove, show thatthey are in core-shell form, with the core comprising diene polymer (andusually occluded SAN) and the shell being SAN copolymer. Said shellcompletely surrounds the particles of said core and is chemically linkedto said particles. Such core-shell copolymers are another aspect of theinvention.

The core-shell copolymers of this invention demonstrate excellent impactstrengths and tensile properties, generally superior to those of similarmaterials in which no chemical link between the core and the shell ispresent. For example, a core-shell copolymer prepared by grafting maleicanhydride on a high rubber graft in the presence of a free radicalinitiator and subsequently melt blending with conventional SAN and a SANcopolymer containing glycidyl methacrylate units is superior in impactstrength to similar blends prepared without the maleic anhydride,despite the fact that the presence of a glycidyl methacrylate-containingSAN generally degrades impact properties. By contrast, the employment ofa maleic anhydride-grafted high rubber graft in combination with anunfunctionalized SAN produces a blend with an impact strength inferiorto those of similar blends not containing the maleic anhydride. Thetensile strength at yield of the composition of the invention is alsonoticeably superior, while tensile strength at break, tensile modulusand elongation are comparable. The compositions of the invention alsoexhibit substantially lower gloss than similar compositions preparedwithout the presence of a chemical bond between the core and the shell.

The invention is illustrated by the following examples. All parts are byweight unless otherwise indicated.

EXAMPLE 1

A mixture of 100 grams (655 mmol.) of vinylbenzyl chloride (mixture ofm- and p-isomers), 42.6 grams (655 mmol.) of sodium azide and 500 mi. ofdimethylformamide was stirred at room temperature for 24 hours. Ethylether, 500 mi., was then added and the mixture was extracted with two300-mi. portions of water. The combined aqueous phases were extractedwith 500 mi. of ethyl ether, and the combined organic layers were driedover magnesium sulfate, filtered and evaporated to yield 105 grams (100%of theoretical) of vinylbenzyl azide. The structure was confirmed byproton and carbon-13 nuclear magnetic resonance spectroscopy andinfrared spectroscopy.

A mixture of 726 grams (6.98 moles) of styrene, 242 grams (4.56 moles)of acrylonitrile, 18.3 grams (115 mmol.) of vinylbenzyl azide, 3.83grams (23.3 mmol.) of azobisisobutyrylnitrile and 1.5 liters of methylethyl ketone was purged with nitrogen for 5 minutes and stirred at 70°C. for 24 hours. It was then cooled to room temperature and precipitatedinto methanol in a blender. The desired vinylbenzyl azide-functionalizedSAN was filtered, washed with methanol and dried at 60° C. In a vacuumoven for 48 hours. The yield was 750 grams (76% of theoretical). Fouriertransform infrared spectroscopy confirmed the presence of the azidegroup.

A mixture of 30 parts of a high rubber graft consisting of 50%polybutadiene and 50% SAN with a 75:25 weight ratio of styrene toacrylonitrile units, in which 40% of the SAN was chemically bound to thepolybutadiene, was tumble blended with 30 parts of theazide-functionalized SAN and 40 parts of a similar unfunctionalized SAN.The dry blend was extruded on a twin-screw extruder at temperatures of120°-220° C., and the extrudate was cooled in a water bath, pelletizedand dried at 80° C. for 4 hours in a recirculating air oven.

It was shown by transmission electron microscopy to be composed ofcore-shell particles with the SAN shell completely surrounding thepolybutadiene core, presumably at least in part as a result of chemicalbonds between the core and the shell.

EXAMPLE 2

A functionalized SAN was prepared by a method similar to that describedin Example 1, with the substitution of 16.3 grams (115 mmol.) ofglycidyl methacrylate for the vinylbenzyl azide. The yield was 790 grams(80% of theoretical). Fourier transform infrared spectroscopy showed thepresence of glycidyl methacrylate units.

A high rubber graft consisting of 50% polybutadiene and 50% SAN with a75:25 weight ratio of styrene to acrylonitrile units, in which 40% ofthe SAN was chemically bound to the polybutadiene, was tumble blendedfor several minutes with 0.25% dicumyl peroxide and 1.5% maleicanhydride. The dry blend was extruded on a twin-screw extruder attemperatures of 120°-220° C. to form an anhydride-functionalized highrubber graft as the extrudate, which was cooled in a water bath,pelletized and dried at 80° C. for 4 hours in a recirculating air oven.

A mixture of 30 parts of the anhydride-functionalized high rubber graft("HRG"), 30 parts of glycidyl methacrylate-functionalized SAN and 40parts of unfunctionalized SAN was prepared under the same conditions. Itwas injection molded into tensile strength and Izod impact strength testspecimens, and the physical properties were determined in accordancewith ASTM methods D638 and D256, respectively, in comparison withseveral controls. The results are given in the following table. SANpercentages are based on total resinous components.

    ______________________________________                                                   Controls                                                                      Example                                                                              A      B      C    D    E                                   ______________________________________                                        HRG preparation:                                                              Maleic anhydride, %                                                                        Yes      No     No   Yes  No   No                                Dicumyl peroxide, %                                                                        Yes      No     Yes  Yes  No   Yes                               Blend:                                                                        GMA-func. SAN, %                                                                           30       30     30   --   --   --                                Unfunc. SAN, %                                                                             40       40     40   70   70   70                                Notched Izod impact                                                                        53       37     43   59   80   85                                strength, joules/m.                                                           Tensile strength,                                                             MPa.:                                                                         At yield     59.6     53.9   55.6 56.8 56.2 56.3                              At break     50.7     52.2   44.2 55.3 51.8 47.7                              Tensile modulus, GPa.                                                                      13.1     13.2   12.9 13.3 12.6 12.8                              Tensile elongation, %                                                                      13.5     7.4    13.9 7.5  8.8  13.3                              Gloss (reflectance)                                                                        33       33     29   90   89   79                                ______________________________________                                    

The results in the table show the superiority of the compositions ofthis invention in impact strength to Controls A and B in which the highrubber graft is not maleic anhydride-functionalized, and thereforecannot interreact with the SAN. The opposite trend is seen when noglycidyl methacrylate-functionalized SAN is present, as shown byControls C-E. The tensile strength at yield of the composition of thisinvention was somewhat higher than those of any of the controls. Tensileyield at break and tensile modulus were comparable and tensileelongation compared favorably to those of the controls. Finally, thecomposition of the invention and comparable Controls A-B havesubstantially lower gloss than Controls C-E.

What is claimed is:
 1. A core-shell copolymer comprising particleswherein the core comprises particles of a diene-maleic anhydridecopolymer and the shell is a copolymer of an ethylenically unsaturatednitrile, an alkenylaromatic compound and glycidyl methacrylate, saidshell completely surrounding and being chemically linked to said coreparticles.
 2. A copolymer according to claim 1 wherein the core alsocomprises a copolymer of an ethylenically unsaturated nitrile and analkenylaromatic compound.
 3. A copolymer according to claim 1 whereinthe diene is butadiene, the alkenylaromatic compound is styrene and theethylenically unsaturated nitrile is acrylonitrile.
 4. A core-shellcopolymer comprising particles wherein the core comprises dieneparticles and the shell is a copolymer of an ethylenically unsaturatednitrile, an alkenylaromatic compound and vinylbenzyl azide, said shellcompletely surrounding and being chemically linked to said coreparticles.
 5. A copolymer according to claim 4 wherein the core alsocomprises a copolymer of an ethylenically unsaturated nitrile and analkenylaromatic compound.
 6. A copolymer according to claim 5 whereinthe diene is butadiene, the alkenylaromatic compound is styrene and theethylenically unsaturated nitrile is acrylonitrile.